The “Pharaoh’s Serpent” chemical reaction involves the thermal decomposition of mercury (II) thiocyanate:^1,2

2 Hg(SCN)₂ (s) + 3 O₂(g) → 2 HgS(s) + C₃N₄(s) + 2 SO₂(g) + CO₂(g)       Equation 1

As the gaseous products permeate the solids formed during the reaction, a column of foam is created that looks like a snake. While this reaction is very interesting to observe, the toxicity associated with this experiment is enough to keep me from using it in the classroom.

However, I recently became aware of a similar “chemical serpent” that can be generated by the thermal decomposition of calcium gluconate:^3,4

CaC₁₂H₂₂O₁₄(s) → 11 C(s) + 11 H₂O(g) + CaCO₃(s)             Equation 2

This reaction can be carried out very simply by igniting a fuel tablet (like those used for camping) and placing calcium gluconate tablets (sold as supplements) on top. Better yet, both reactants and products are completely benign. Because of this, I had to try the reaction out for myself. When doing so, I noticed quite a few interesting details (see Video 1).

Video 1: The Chemistry of Pharaoh's Serpent Explained!, Tommy Technetium YouTube Channel, February 19, 2024.

There is a LOT of chemistry involved in this experiment! Consider, for example, the chemical reaction that occurs as the fuel tablet burns. These tablets are made of hexamine (C₆H₁₂N₄), which combusts according to:⁵

C₆H₁₂N₄(s) + 9 O₂(g) → 6 CO₂(g) + 2 N₂(g) + 6 H₂O(g)                  Equation 3

The decomposition of calcium gluconate (Equation 2) also displays a variety of interesting physicochemical phenomena. First, is the creation of the serpentine black foam, which fades to grey and white over time. Second, the foamy snake emits an orange light when it is black, but white light as its color fades to grey or white. I found a study published on the thermal decomposition of calcium gluconate and noted that all these features can be explained by the chemistry that occurs. As noted in the video, the carbon that is produced (Equation 2) is what gives the foam its black color. This carbon is also responsible for the orange light that is emitted, because carbon glows an incandescent orange when heated. The observed shift in the color of the foam from black to white can also explained by noting that the carbon is removed as it is converted into CO₂ (Equation 4) leaving behind white CaCO₃:

C(s) + O₂(g) → CO₂(g)        Equation 4

It is mostly gaseous water (Equation 2) that permeates the solid products to cause the foam, but I wonder if the CO₂ produced (Equation 4) also contributes. The bright white light emitted from the foam results as CaCO₃ is converted into CaO, which is also known as lime:

CaCO₃(s) → CaO(s) + CO₂(g)           Equation 5

And when lime is heated to high temperatures, it emits bright white light: It’s limelight!

Adding together Equations 3-5 we can write an overall reaction for the reaction between calcium gluconate and oxygen to form gaseous water, carbon dioxide, and calcium oxide:

CaC₁₂H₂₂O₁₄(s) + 11 O₂(g) → 12 CO₂(g) + 11 H₂O(g) + CaO(s)       Equation 6

I found it interesting to use various thermodynamic data (Table 1) to look at the reactions described in Equations 3-5 in more detail. Using the familiar equations:

           Equation 7

                     Equation 8

ΔG = ΔH - TΔS                                              Equation 9

the values of

,

, and

 for reactions 3-5 displayed in Table 2 are obtained.

Table 1: Thermodynamic data of the species in Equations 3-56-8

Table 2: Thermodynamic values for Equations 3-5 at T = 298.15 K

While the reaction described in Equation 3 is only slightly exothermic (-104 kJ mol^-1), ΔG for this reaction is very negative. This is likely because the associated increase in entropy for this reaction is very positive (+1920 J mol^-1 K^-1). Perhaps some of the free energy from the reaction in Equation 3 provides energy to create the foam. It is interesting to note that the reaction described in Equation 5 is not spontaneous at normal temperatures. The temperature at which this reaction is expected to become spontaneous can be estimated by insertion of ΔH° and ΔS° for Equation 5 into Equation 8:

T ≈ ΔH/ΔS                  Equation 8

Upon doing so, a value of T = 1120 K is obtained. The energy from the combusting hexamine tablet certainly contributes to a higher temperature. It could also be that the large free energy release from Equations 3 and 4 could account for heating the foam to higher temperatures, allowing the CaO product to emit limelight!

This reaction has so many fantastic features in addition to the creation of the foamy serpent. It emits different colors of light and displays a subtle color change. The reaction is easy to set up, to carry out, and clean up. It connects to five different chemical reactions and a variety of topics in chemical thermodynamics. I highly recommend trying this experiment out for yourself. If you do, be sure to let me know what kinds of things you observe, and what kind of chemical topics you think about as you watch the reaction play out.

Happy Experimenting!

References:

1. https://pubs.acs.org/doi/epdf/10.1021/ed017p268
2. https://web.archive.org/web/20120201221503/http://chemistry.about.com/od/fireworksprojects/a/pharaohs-snakes.htm
3. https://melscience.com/US-en/articles/gluconate-snake-experiment/
4. Labuschagne; F. J. W. J.; Focke, W. W. Metal catalysed intumescence: characterisation of the thermal decomposition of calcium gluconate monohydrate J. Mater. Sci. 38, 2003 1249–1254.
5. Merritt, J. R.; Herington, L.; Jones, S. B.; Sayed, Y. Analysis of Hexamine Combustion Am. Ind. Hyg. Assoc. J. 52, 1991, 30-33.
6. Di, Y.-Y.; Zhang, G.-C.; Lui, Y.-P.; Kong, Y.-X.; Zhou, C.-S. Crystal structure and thermodynamic properties of the coordination compound calcium D-gluconate Ca[D-C6H11)7]2(s) J. Molec. Sruct 1225, 2021, 1-10.
7. Eisermann, W.; Johnson, P.; Conger, W. L. Estimating Thermodynamic Properties of Coal, Char, Tar, and Ash. Fuel Processing Technology, 3, 1980, 39-53.
8. Ebbing, D. D.; Gammon, S. D. General Chemistry, 11th ed.; 2017, Cengage Learning.

Note: Both the camp fuel pellets and the calcium gluconate supplement tablets can easily be found online and in stores. One online option for the fuel pellets is Coghlan's Fuel Stove Tablets on Amazon. An option for the calcium gluconate tablets is the Nature Made brand on Amazon.